We will let Hannah Arendt, one of the greatest political philosophers of our time, reply to this last question: 1
The miracle that saves the world, the domain of human affairs, from normal, "natural" ruin, is finally the fact of birth, in which is rooted ontologically the faculty to act. In other words it's the birth of new men, the fact that they begin anew, the action they are capable of by right of birth. Only the total experience of this capacity can bestow on human affairs faith and hope, these two essential characteristics of existence which Greek antiquity completely misunderstood, setting aside vowed faith in which they saw a very rare and negligible virtue, and ranking hope among the number of pernicious illusions of Pandora's box. It is this hope and faith in the world which have found, without doubt, their most succinct and glorious expression in the little phrase of the Gospel announcing the "good news": "A child is born to us."_______________________________________________
1 Cf. Hannah Arendt, Condition de l'homme moderne (Paris: Calman-Levy, 1958, reprinted in 1988) 314.